Page 613 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

568 Fundamentals of Water Treatment Unit Processes: Physical, Chemical, and Biological

sizes in the UF range, for example, 1–10 nm, have not Product flow: See permeate.
been successful. Microporous membranes are suscep- Rack: See membrane technology.
tible to plugging (Cheryan, 1986, p. 29). Recovery: Ratio of product flow to feed flow. Typically, one
Module: See membrane technology. pass through an RO membrane for seawater gives
Molecular weight cutoff (MWCO): The pore sizes of a 40%–60% recovery (of permeate), that is, about
membrane may have a distribution. The MWCO is 50% of the concentrate is brine. An RO membrane
smallest molecular weight of a test molecule that will with brackish feed water gives typically 70%–80%
not pass the pores of the membrane to a significant recovery.
extent (Wiesner, 1993, p. 5). Molecules having Reject flow: See concentrate flow.
MW MWCO will not pass through the membrane. Reverse osmosis: When pressure is applied to the saline
A portion of the molecules having MW < MWCO water side of a semipermeable membrane that is in
will pass, depending on the size of the molecules and excess of the osmotic pressure that is due to the
the pore-size distribution. saline water, water will flow from the saline water
Nanofiltration: Membrane filtration that removes many side to the permeate side. While osmosis occurs by
organic molecules, and some larger ions, particularly diffusion from the pure water side to the saline water
ions that are complexes. side, ‘‘RO’’ causes water to flow in the reverse dir-
Osmosis: The diffusion of water from the pure water side of a ection (see also ‘‘osmosis’’ and ‘‘hyperfiltration).
semipermeable membrane to the saline side. Since the flow of water from the saline side to the
Osmotic pressure: If a semipermeable membrane separates a permeate side is proportional to the pressure differ-
pure water and a saline solution, the water level in ential (after subtracting the osmotic pressure), one
the saline solution side will rise to a level that is the may infer that the Poiseuille law applies. (There may
osmotic pressure, p, of the saline solution. The be some debate on this point.)
osmotic pressure may be calculated by the van’t Screen filter: A filter in which suspended material larger than
Hoff equation for dilute solutions. the openings is retained on the surface (Cheryan,
Package: The membrane element that includes whatever 1986, p, 27).
ancillary parts are necessary such that the system Silt density index: An empirical test developed for mem-
works as a technology (as used here for descriptive brane systems to measure the rate of fouling of a
purposes; the term is not one that is used commonly 0.45 mm filter pad by the suspended and colloidal
in the industry). particles in a feed water. This test involves the time
Permeability: A characteristic of the flow resistance of a required to filter a specified volume of feed at a
membrane, defined in terms of Darcy’s law, Equa- constant 30 psi at time zero and then after 5, 10,
tion E.5, v ¼ K(dh=dz), in which K ¼ krg=m, and 15 min of continuous filtration. Typical RO
Equation E-4. Alternatively, the Poiseuille equation, element warranties list a maximum SDI of 4.0 at
Equation D.11, also describes the same, only it is 15 min for the feed water. If the SDI test is limited
in terms of viscous flow through a pipe, that to only 5 or 10 min readings due to plugging of the
2
is, dp=dz ¼ 32mv(avg)=d ; rearranged, v(avg) ¼ filter pad, the user can expect a high level of fouling
2
2
(d =32m)(dp=dz) ¼ (d rg=32m)(dp=dz). In the latter for the RO. Deep wells typically have SDIs of three
2
2
case, K ¼ d rg=32m and therefore, k ¼ d =32. See or less and turbidities less than one with little or no
also intrinsic permeability. pretreatment. Surface sources typically require pre-
Permeate: The flow of water through the pores of the mem- treatment for removal of colloidal and suspended
brane. solids to achieve acceptable SDI and turbidity val-
Permeate flow: Flow of permeate, Q P , that is, water that has ues. Foregoing is described in, SDI–Silt Density
been transported across the membrane, leaving con- Index Examined and Explained, Industrial Water
taminants behind, as illustrated in Figure 17.1. Treatment Consulting, 2010, http:==www.iwtc.com.
Plate-and-frame: A configuration of membrane sheets in au=resources=IWTC_SDI_examined_and_explained.
which the feed flow is between two membrane pdf.
sheets. Spacers of some material are located on the Solute: The dissolved solid portion of a solution. Most often,
other sides of the respective membrane sheets. The the term is used in reference to a particular species of
pattern is repeated such that perhaps several hundred dissolved solids, for example, Na , sucrose, etc.
þ
sheet–spacer pairs comprise the membrane system. Solute rejection: The fraction of solute that is ‘‘rejected’’ by
Polarization: The point at which the current density the membrane and is carried out in the reject flow. The
2
(amps=m ) is high enough to dissociate the water term has not been applied to hollow-fiber membranes
molecule, resulting in the formation of OH and (but could be defined based on the flow of water for
H ions (Meller, 1984). hydraulic flushing over, say a 24 h period).
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Pressure vessel: Usually a tube that houses one or more Spiral wound: A higher area of membrane surface area per
membrane elements (see also module). unit volume is obtained by configuring an array of